It is widely accepted that there are at least three significant levels of high frequency power combining. At a first level, increased power output can be obtained through the combination of several devices on a single chip carrier (packages, pins, headers or the like). At this level, distances are maintained small with respect to a quarter wavelength so that the composite device acts as a single device electrically. At an intermediate level, modulator circuitry may be employed wherein several devices are summed by circuit means that determines their performance (such as a push-pull amplifier) prior to the delivery of their output to a transmission line for further combination in a totally different form of circuit. Finally, gross or large-scale power combining involves a multiplicity of devices, transmission lines or antennae summed in a manner which does not discretely limit the number of devices. The first of the aforementioned levels has received considerable effort and innovation over the years, including series and/or parallel power combining of devices on diamond heat sinks and metal headers. At the gross level, considerable effort and support has produced some innovation. For example, a large-scale power-combining device is disclosed in U.S. Pat. No. 3,931,587, issued to Robert S. Harp and Harry L. Stover, entitled "Microwave Power Accumulator". This patent is the property of the assignee herein.
The second of the aforementioned levels, the modular level of power combining, has not yet received significant attention. By definition, the modular level infers a class of amplifiers/oscillators that requires more than one device to function properly in a given, desirable mode. Push-pull amplifiers and balanced amplifiers are examples of this class of combiner; the performance of the system or module is unlike the performance of a single device in some desirable respect.
Push-pull power amplifier/combiners, in which the power outputs of two separate devices (e.g., FETs, bipolar transistors, IMPATT or GUNN diodes) are in some way combined to produce the output transformer primary current, may be classified according to their mode of power combination. In a class-A amplifier/combiner, the two devices, simultaneously "on", are run out of phase, producing a standing wave across the primary which oscillates about a center null point. Contrariwise, in class-B operation, the power devices are switched on and off alternately to deliver a full-wave rectified pulsating current to the center of the primary. Significant operational advantages (in terms of r.f. power output, d.c. to r.f. efficiency, and transistor junction temperature) are achieved by class-B operation as a result of the fact that twice the current swing (and, consequently, four times the instantaneous power) can be achieved in class-B operation for a given FET (or equivalent device) power swing. When no input signal is present in the class-A amplifier, the maximum d.c. power is dissipated and, at maximum output r.f. voltage swing, at most half of the d.c. power is converted to a.c. Conversely, when no input signal is present in a class-B amplifier, no current flows and, hence, no power is dissipated. This feature is very significant when the dynamic range of the amplifier/combiner is an important system consideration.
Consistent with the foregoing, it can be shown in theory that class-B operation delivers twice the r.f. power of class-A operation and at a higher efficiency. Device junction temperature rise in class-B is lessened by a factor of 1.div.1.83 despite the generation of twice the r.f. power. Thus, class-B push-pull operation is eminently more suited for power combining applications than class-A single-ended or class-A push-pull. A large-scale combiner incorporating class-B modules would require half an many power modules for a specified peak power. Likewise, for a large scale combiner operating at thermal limits, approximately one fourth as many modules are required. Additionally, prime power requirements are reduced in either application (due to the improved d.c. to r.f. conversion).
Although the advantages of the class-B power amplifier are well known and have been commonly achieved at lower frequencies, class-B operation at microwave frequencies has been severely hampered by the distortion encountered at gigahertz-range frequencies resulting from crosstalk between the active devices. This cross-talk is a result of imperfections in achieving a true voltage null at the center of the primary. The sensitivity of class-B operation results in part from the continual switching of the two power devices at gigahertz-range frequencies. Fabrication of a transformer primary for gigahertz-range frequencies with a reliable center tap has been extremely difficult to achieve and has led to alternative approaches including the less-desirable class-A mode and the use of 3 db couplers and the like. An additional difficulty at high frequencies results from the extreme sensitivity of inductive coupling to primary-secondary spacing. Such sensitivity limits the ability to vary the mutual coupling to adjust power device loading for higher power and efficiency. The ability to make such an adjustment increases the efficiency of the module; a slight misadjustment in the coupling can cause efficiency to degrade, resulting in greater power dissipation in the transistor and, often, device burnout.